The chances of seeing a $300 million redevelopment of Johnsonville Mall before 2015 look slim if the New Zealand Transport Agency does not chip in for a $14m upgrade of local roads.

Operator DNZ has been granted consent to increase the mall from about 10,000 square metres to a 32,000sqm two-level shopping centre.

But those consents are contingent on transport flows improving along Johnsonville, Broderick and Moorefield roads - known as the Johnsonville Triangle - which border the mall site.

The estimated $14m cost of the work was to be funded by DNZ, the council and the NZTA.

Last year, the council brought forward plans to spend $5.7m on the project over the next three years in the hope it would allow work to begin on the mall this year.

But when the agency announced its national land transport programme of works for 2012-15 six months ago, it did not approve any funding for the Johnsonville Triangle.

The council had been hoping for about $2m to $3m in government funding.

Instead, the agency gave the project "reserve" status and told the council that funding could still be approved within the current period if it could be convinced of the project's economic benefits.

Transport portfolio leader Andy Foster said the council and DNZ has been working for months on a funding proposal to put to the transport agency.

In the council's view, NZTA needed to be on board as a funding partner before the road works and subsequent mall redevelopment could happen, Mr Foster said.

"We've put in the money from our budget. We obviously have significant budget pressures and ... can't magic any extra funds out of our hat, because we don't have that hat."

He was confident the agency could be convinced to come on board before 2015. If the agency was to say no, then the council would "keep arguing" with it, he said.

"In this instance, you've got economic development directly relying on roading investment in Johnsonville, so you would think that, when that assessment is done [by NZTA] it would look pretty good."

NZTA central region director Jenny Chetwynd did not say what the Johnsonville Triangle's funding prospects were at this stage.

The agency was looking for an "integrated solution" that improved transport for pedestrians, cyclists, public transport users and local road users.

"It's also important that the project delivers transport benefits over and above that of mitigating the effects of commercial development, to ensure that we are investing in an overall improvement in people's journeys."

DNZ chairman Tim Storey said the company had no knowledge of the roading arguments. DNZ was waiting for agreement from all tenants, and for an upturn in the economy, before committing to the redevelopment.